A ceramic capacity sensor element 1 for pressure sensing is usually constructed of two parts, see FIG. 1. These parts are a stable circular base plate 3 having typically a diameter of 20–30 mm and a thickness of 4–5 mm and a thinner circular plate 5, also called a diaphragm, having a movable central portion and mounted at one of the large surfaces of the base plate 3 and joined thereto by means of for example glass joints 6 at the circular edges of the diaphragm and base plate. The diaphragm 5 has the same diameter as the base plate and has a thickness which is adapted to the magnitude of the load to which the diaphragm is to be subjected. The change of the position of the central portion of the diaphragm 5 is detected as a change of the electrical capacitance between two electrodes 7, 9 of e.g. gold, which are located in parallel with and opposite each other and which are coated using thin film methods on the facing, inner surfaces of the base plate 3 and the diaphragm 5 respectively. The sensor element 1 can be used for different types of measuring pressures, in which the variable searched for is the pressure Pmeas, which acts on the free, outer surface of the diaphragm 5 facing away from the base plate 3. The measurement is always made in relation to a reference pressure Pref in some form. Pressure sensors can be classified according to the way in which the reference pressure is formed. Thus the following types are obtained:
“a gauge-sensor”if Pref = the atmospheric pressure“an absolute sensor”if Pref = technical zero pressure“a differential sensor”if Pref = a second pressure to be measured.
Of course, in a more strict sense all these types use a differential or relative measurement.
A gauge-sensor uses a circular hole 11 through the base plate 3 from the free surface thereof into the cavity 13 formed between the inner surfaces of the base plate 3 and the diaphragm 5 as a channel for conducting the ambient atmospheric pressure to the inner side of the diaphragm 5. If the pressures on the two sides of the sensor element of a gauge-sensor are equal and particularly the pressures on the two sides of the diaphragm 5 are equal, i.e. if Pmeas=Pref=the atmospheric pressure, the diaphragm 5 will rest in a flat position. If the pressure to be measured is larger than the reference pressure, i.e. if Pmeas>Pref, where Pref=the atmospheric pressure, the diaphragm 5 will bend in towards the base plate 3 and the capacitance between the electrode surfaces 7, 9 will be changed, which is electrically detected.
If the inner cavity 13 between the base plate 3 and the diaphragm 5 and the channel 11 is evacuated from air and other foreign gases and is closed by e.g. a tin plug, a situation is obtained in which the pressure to be measured is for example of the magnitude of order of an ambient pressure, i.e. Pmeas=the atmospheric pressure, and the reference pressure is substantially equal to zero (vacuum or technical zero pressure), i.e. Pref=0. Here an exact zero pressure is not considered but a zero pressure which can be produced technically, i.e. of the magnitude of order of 10−6 torr.
The diaphragm 5 then bends in towards the base plate 1, since the pressure Pmeas to be measured, which then e.g. is approximately equal to the atmospheric pressure, is larger than the reference pressure Pref. If the pressure Pmeas to be measured is increased to become larger than the atmospheric pressure, the diaphragm 5 bends even more in towards the base plate 3. If the pressure Pmeas to be measured instead is reduced from the atmospheric pressure in order to approach the vacuum range, the diaphragm bends less and less in towards the base plate. When the pressure to be measured reaches a technical zero pressure, i.e. Pmeas=Pref=technical zero pressure, the diaphragm 5 will rest in a flat position. This type of absolute sensor is apparently based on the fact that the inner reference pressure Pref is maintained intact and is maintained at a substantially constant, very low pressure during a long time period.
If e.g. air and/or other gases slowly leak into the sensor element 1 into the reference cavity 13 of the sensor element, the sensor element will gradually lose the possibility to operate as an absolute sensor. Leakage can take place by for example permeation of gas molecules through the ceramic material in the base plate and diaphragm, through the glass joint or through the tin plug which closes the channel 11. If the reference cavity 13 has a small volume, the increase of pressure therein can occur rapidly, which can be counteracted by increasing the volume of the cavity in order to for example also comprise a room on the top side of the base plate, which results in a more complex structure. The cavity can also be provided by some form of device, which maintains a correct level of the reference pressure Pref during a longer time. Such a device can e.g. be a “non-evaporable getter” (NEG), i.e. in principle a body of a gas absorbing material. A suitable choice can be a porous sintered material such as e.g. Zr and/or an alloy of Zr, V and Fe. The material can then act as a small in-situ vacuum pump, which absorbs foreign gases in the reference cavity. For an NEG integrated in the reference cavity a high qualitative reference pressure Pref of the magnitude of order of 10−8–10−10 torr or lower is obtained.
An NEG is activated by specific high vacuum/temperature processes. If the activating process is executed for a sensor element, which mainly is under atmospheric pressure, it can be executed e.g. in the following way, see FIGS. 2 and 3. The sensor element 1, which is here provided with a stabilizing ring 15 mounted at the margin region of the diaphragm 5, see the published International patent application WO 95/28624, is mounted in a recess 17 in a jig 19, which is mounted on top of a heating plate 21. A channel 22 connects the free surface of the diaphragm 5 to a high vacuum pump, not shown. A tip-off tube 23 is attached in a recess in the free surface of the base plate 3 having a connection to the channel 11. The tip-off tube 23 is connected to the high vacuum pump so that the inner surface of the diaphragm will also be subjected to the vacuum. Thereby the diaphragm will be located in a flat position all the time during the activation and closing and will be exposed to minimum mechanical stresses. The sensor element 1 is slowly heated to temperatures about 200–300° C., by energizing the heating plate 21. Gas molecules inside the reference cavity 13 and at the surfaces of the cavity are “shaken” and thus detached from the surfaces and are then pumped out by the high vacuum pump. The tip-off tube 23, which can be made of e.g. copper or glass, is closed by heating it locally to a very high temperature and then pinching it off, when this so called bake-out procedure is finished.
A closed getter tube 25 is mounted in another low recess on the free surface of the base plate 3, the recess being connected to a second channel 27 extending in to the reference cavity 13. The getter tube can made be of e.g. copper or preferably of glass and contains an NEG 28 having the shape of a rod, which is located in a transverse position inside the getter tube 23 and has a resistive inert wire 26 of e.g. platinum wired around it. The platinum wire 26 is introduced in an electrically isolated way through the getter tube, for example molten into glass, in the case the tube 25 is made thereof, so that an electric current can be conducted through the wire 26. The resistive wire 26 can also be integrated in the NEG-element 28 and then be located inside it. The capability of the NEG-element 28 of in-situ pumping (strictly absorbing) gas molecules is activated by the gradual heating. When the bake-out approaches its end, first the tip-off tube 23 is closed, see 29 in FIG. 3, and then a short, intense final activation of the NEG-element 28 is executed. A current is now conducted through the platinum wire 26, which then starts to glow and intensely increases the temperature of the NEG-element 28. This temperature increase of the NEG-element 28 results in a final activation of the NEG-element, which thereby increases its capability of absorbing foreign undesired gases in the reference cavity.
The obtained reference pressure is of the magnitude of order of 10−8–10−10 torr. The NEG-element 28 will maintain its function also when the entire sensor element 1 and then also the NEG-element included therein has cooled to ambient temperature. The function is maintained until the NEG-element 28 has been saturated with foreign gases originating from e.g. leakage into the cavity. If impermeable ceramic materials are used, the saturation time of the getter will clearly exceed the commercial technical lifetime of the element. A reference cavity in which e.g. an NEG-element according to the discussion is used provides a high qualitative pressure having a long lifetime. However, the process, which has been described above, has a number of complications and disadvantages as to the production method, the design and the method of operation.